The conventional transmission incorporates a number of planetary gears which are selectively coupled between the input and output shafts for changing the velocity ratio of the transmission. However, the conventional transmission results in an inefficient transfer of torque between the input and output shafts since the prime mover must be uncoupled from the transmission output shaft while the planetary gears are switched. Further, since engine speed must vary in each gear ratio to affect speed change of the output shaft, the efficiency of the engine cannot be maximized for any particular operating condition. Therefore, many attempts have been made to provide a transmission whose velocity ratios are infinitely variable over the velocity ratio continuum.
For instance, Beschkine (U.S. Pat. No. 2,239,313) teaches a gear system incorporating continuously-meshing non-circular gears. As shown in FIG. 3 of the patent, the gear system comprises a driving shaft P including a plurality of non-circular gears 1, 2, 3, 4, and a driven shaft R parallel to the driven shaft P including a plurality of non-circular gears, 1′, 2′, 3′, 4′ meshing with the gears 1, 2, 3, 4. The driven gears 1′, 2′, 3′, 4′ are coupled successively to the driven shaft R by electromagnetic clutches for a respective portion of the interval of revolution of the driven shaft R so that the velocity ratio of the gear system is dependent upon the gear ratios of the gears 1–1′, 2–2′, 3–3′, 4–4′ over their respective coupling intervals. Consequently, the velocity ratio of the gear system is varied by simply changing the angular position of the gears 1′, 2′, 3′, 4′ during their coupling intervals. However, as the clutches must be activated each revolution of the driven shaft R, the clutches must be relatively small to be used for high speed applications, thereby limiting the torque which can be carried by the gear system.
Kerr(Canadian Patents 990,103; 1,000,526; 1,031,190; U.S. Pat. Nos. 3,919,895: 4,055,091) teaches variable output transmission incorporating square-wave generators for facilitating changes to the velocity ratio. Each transmission comprises a pair of non-circular driving gears coupled to an input shaft, and a pair of non-circular driven gears which continuously mesh with the driven gears. The velocity ratio profile of each non-circular gear pair resembles a triangular or saw-tooth wave. The rotational outputs of the two non-circular gear pairs are combined-through a differential to provide a differential output having a square-wave velocity ratio profile. The differential outputs of a number of such differentials are combined together through one-way overrunning clutches to a provide a velocity ratio which is infinitely variable in accordance with the relative angular displacement of the driving gears. The variable output transmissions taught by Kerr represented a significant advance over the prior art. However. overrunning clutches can only transfer energy in a single direction, thereby precluding engine drag. Further, as the variable output transmissions only amplified the positive or negative periods of the square-wave velocity ratio profile, the efficiency and maximum kinematic range of the transmissions was limited.
Takahara (U.S. Pat. No. 4,944,718) teaches an angular velocity modulating device which, as shown in FIGS. 1 to 3 of the patent comprises a first rotatable shaft 24 rotatably coupled to a stationary first frame 61; non-circular internal gears 11 mounted on the first shaft 24; a second parallel shaft 34 rotatably mounted on a rotatable second frame 62; second non-circular gears 21 meshing with the first non-circular gears 11 and fixed on the parallel second shaft 24; third non-circular gears 31 meshing with the first non-circular gears 11 and mounted on the parallel third shaft 34 through an overrunning clutch 37; an input shaft 44 including a circular gear 27 for rotating the second shaft 24; and an output shaft 54 including a circular gear 54 driven by the third shaft 34. Since the velocity ratio of the device is varied by changing the angular displacement of the second frame 62 relative to the first frame 61, rapid changes in velocity ratio would be difficult to attain since the angular displacement of the second frame 62 could only be changed by also moving the second shaft 34 and the mass of the accompanying non-circular gears 34. Further, as discussed above, the overrunning clutches preclude engine drag and reduce the efficiency and maximum kinematic range of the device.
Recently, Pires (U.S. Pat. Nos. 5,226,859; 5,334,115) disclosed an infinitely variable transmission which eliminates the need for overrunning clutches. As shown in FIGS. 2, 3 and 4 of the '859 patent, the transmission comprises an input shaft 5, a planetary rotor 29 connected to the input shaft 5 through a front plate 5′, a first pair of crank arms 8a, 8c rotatably coupled to the planetary rotor 29, a second pair of crank arms 9b, 9d rotatably coupled to the planetary rotor 29, and an index plate 7 which incorporates slots for receiving an end of the crank arms. The index plate 7 is supported on an index slide 6 which allows the index plate to move laterally of the shaft 5. The transmission also includes four planar differential gear sets, each set comprising an internally-toothed ring gear 12 coupled to one of the crank arms, a pair of pinions 14 meshing with the ring gear 12, and a sun gear 15 meshing with the pinions 14. The sun gear 15 of each differential gear set is connected to a reaction gear 16 which meshes with an internally-toothed stationary commutator gear 28.
In operation, when the input shaft 5 rotates, the planetary rotor 29 is forced to rotate, causing the crank arms to drive the index plate 7 about its own axis of rotation, as defined by the index slide 6. If the index plate 7 is eccentric to the axis of the planetary rotor 29, the crank arms oscillate about their own axes while orbiting the planetary rotor 29. The amplitude of oscillations is a function of the eccentricity of the index plate 7. The rotational oscillations are delivered to the differential gear sets by the ring gears 12. As shown in FIG. 7 of the patent, the commutator gear 28 includes teeth only around half of the inner circumference of the gear, so that the reaction gears 16 rotate freely one half of a rotational cycle of the input shaft 5. Consequently, when the “desired” polarity of oscillation is present at the crank arm, the commutator gear 28 provides a supplemental rotational input to the differential, whereas when the “undesired” polarity of oscillation is present, the commutator gear 28 prevents the oscillation from reducing the output of the transmission.
Although the transmission taught by Pires addresses the problems imposed by overrunning clutches on efficiency and kinematic range, the transmission is quite complex. Further, it is believed that the oscillating crank arms will produce undesirable fluctuations in the velocity ratio of the transmission. Accordingly, there remains a need for an infinitely-variable all gear transmission which has an enhanced kinematic range, is capable of making rapid changes in velocity ratios, and can take advantage of engine drag.
Further, conventional non-circular gears employ standard involute-shaped gear teeth. Although the involute-shaped gear teeth are acceptable for use with circular gears, involute-shaped gear teeth when used on non-circular gears cause the contact ratio between the gears to continuously vary. These variations in contact ratio cause excessive gear noise. Also, the contact ratio of involute-shaped teeth even on circular gears rarely reaches 2.0. Consequently, the load which can be carried by the gears is limited. Although the contact ratio may be increased by twisting the gear teeth, twisted gear teeth produce point contact which creates Hertzian stress. Accordingly, there also remains a need for non-circular gears having a constant contact ratio which is preferably at least 2.0.